Conventional barcode scanners use single color illumination consisting of a red light emitting diode (LED). This minimizes the cost of the scanner but limits the range of colors that can be used in the printed barcode and be detected with the scanner. A similar problem occurs when using the scanner to read digital watermarks.
While conventional barcode scanners typically use red illumination, newer barcode scanners are increasingly moving to white LEDs as illumination for their sensors as opposed to the traditional red LED illumination. The rationale behind this change is that red illumination can be more stressful on the eyes when used for long periods. Red is also more distracting because it does not blend in with the natural ambient light in the room. However, to save on costs the scanner manufactures maintain a monochrome sensor on their scanning devices. The combination of a monochrome sensor and only white illumination means that only a luminance changes can be detected by the scanner as opposed to changes in chrominance, such as changes in blue color direction in the chrominance plane, when using red illumination.
Thus, when implementing image signal coding in scanning hardware with white illumination and a monochrome sensor, the use of color to convey data signals is more limited, as the monochrome sensor will only capture luminance changes under white illumination. For digital watermarking applications where imperceptibility is important, digital watermarks encoded by modulating luminance tend not to be acceptable as they can be more visible than those encoded by modulating chrominance. Where imperceptibility of the data signal encoded in the image is more important, it is preferred to encode the data signal by modulating one or more colors in a chrominance plane. Even where limited to encoding in luminance, color values in an image can be modulated so as to impart a signal in luminance. These color values have luminance and chrominance components, and luminance is modulated by scaling a color vector to increase or decrease its luminance component. See, for example, U.S. Pat. No. 6,590,996, where color values of image signals are adaptively modulated to have reduced visibility yet yield detectable modulation of luminance.
By increasing the number of color LED's used in image capture, a greater range of printed colors can be used in printed barcodes or digital watermarks. This has the added benefit of enabling encoding of auxiliary data signals in a chrominance plane. In particular, to reduce visibility of digital watermarks in host images, digital watermark encoding is performed in one or more colors within a chrominance plane (also called color planes, color channels or color direction). For example, one such approach modulates a host image primarily in the cyan channel to greatly reduce the visibility of the embedded watermark. Further, encoding signals in multiple chrominance channels provides additional benefits in imperceptibility, robustness and detection. An example is encoding out-of-phase signals in at least two chrominance channels. In CMYK printing, for example, changes for encoding digital signals are introduced in the Cyan and Magenta ink channels, and these changes are detected in red and green channels. Cover image content is reduced by subtracting the chrominance channels in a detector. See U.S. Pat. No. 8,199,969, and US Patent Application Publication 20100150434, which are hereby incorporated by reference. In these types of techniques, the use of color LED's in the scanner enables the watermark signals to be extracted and combined from two or more chrominance channels.
The addition of illumination in other wavelengths enables scanners to be used to read still further types of signals. For example, a color near infra-red (NIR) LED could be added to read signals encoded in the K channel in objects printed with CMYK printers. This allows the scanning equipment to exploit out-of-phase encoding in which one of the signals in encoded in the K channel and an out-of-phase signal is encoded in an opposite direction by scaling luminance of CMY channels to offset the change in luminance in the K channel. This out-of-phase encoding reduces visibility as the luminance changes encoded in the K channel are offset by the luminance changes in the CMY channels. CMY inks are transparent to NIR, so the digital watermark is read from K channel by capturing the image under illumination of the NIR LED. See, for example, U.S. Pat. Nos. 6,721,440 and 6,763,123, which are hereby incorporated by reference.
Scanners that use white illumination and a monochrome sensor normally will not be able to detect signals encoded in these other channels. Instead, only encoding in luminance is detectable. This may be suitable for some applications. However, where the data signaling is preferably implemented to minimize visible changes to the host image, luminance watermarking tends to be inferior to chrominance watermarking. From the standpoint of the sensitivity of the human visual system, changes to certain colors in chrominance channels are less noticeable to humans than changes in luminance.
In order to detect with white illumination, manufacturers need to update their scanners to a color sensor or some other means to separate color components of the captured image. For lower cost scanners, a full color video sensor adds cost to the scanner and triples the bandwidth of data from the sensor to the detector (e.g., every frame typically consists of 3 or more components (such as RGB), as opposed to a single component in monochrome sensors).
To provide a broader range of signal capture, one solution is to have a series of different wavelength light sources (e.g., LEDs) that are synchronized to the capture of frames by a monochrome sensor. This allows frames to be illuminated by a single wavelength. For some types of image based data codes, like digital watermarks that are repeated across the surface of a printed object, it is sufficient to illuminate a chrominance based watermark signal for a portion of a frame, as the data signal is fully recoverable from a portion of the frame. If these light sources are flashed quickly enough, they give the illusion to the user of white illumination. When the combination of different wavelength light sources are flashed fast enough (e.g., 200 Hz or more), the illumination appears white with no visible flashing or blinking perceived by the user. This type of controlled lighting can be used in combination with a monochrome sensor and yet capture chrominance information to detect or recognize signals in chrominance channels. As long as acquisition time can be short, the periods for illuminating the sources of different wavelengths can be configured to synch to multiples of the video rate. Various examples of configurations of lighting and capture are provided below.
While the above discussion primarily provides examples of digital watermark and barcode signaling, the techniques can be applied to other forms of image based coding and scanning of such coding from objects. Further, the techniques also apply to signal recognition in visual media, such as pattern recognition, computer vision, image recognition and video recognition. Various objects, such as goods or packaging for them, may be created so as to be composed of color combinations and/or include various patterns that constitute signals for which these techniques offer enhanced recognition capability. Objects can, for example, be discriminated from background clutter. Likewise, logos can be discriminated from other package or label image content.